Java While Loop - Syntax, Flowchart and Examples
⏱️ 7 min read • Beginner Level • Lesson 26
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The while loop in Java is used to execute a block of code repeatedly
as long as a given condition remains true. It is an entry-controlled loop,
which means the condition is checked before the loop body executes.
Before learning the while loop, you should understand
Java if statement,
Java operators, and
Java variables.
What is While Loop in Java?
A while loop is a looping statement that repeats a block of code while
the condition is true. If the condition is false at the beginning, the loop body will
not execute even once.
-
The condition is checked before every iteration.
-
The loop body executes only when the condition is
true. -
A proper update statement is usually required to avoid an infinite loop.
Syntax of while Loop
A while loop usually has three important parts: initialization, condition, and update.
initialization;
while (condition) {
// loop body
update;
}while must be a boolean expression.
For example, i <= 5, count > 0, or isRunning == true.
Flowchart of while Loop
The following diagram shows how a while loop works. Java checks the condition first. If the condition is true, the loop body executes. After that, the condition is checked again.
How does it work?
- Java first checks the condition inside the
whilestatement. - If the condition is
true, the loop body executes. - After the loop body executes, the update statement changes the loop variable.
- Java checks the condition again before the next iteration.
- If the condition becomes
false, the loop stops.
Real-World Uses of while Loop
- Reading user input until a valid value is entered.
- ATM menu systems.
- Game loops.
- Processing files and records.
- Repeating tasks until a condition becomes false.
Java while Loop Example
The following program prints Hello five times using a while loop.
public class WhileDemo {
public static void main(String[] args) {
int i = 1;
while (i <= 5) {
System.out.println(i + ". Hello");
i++;
}
}
}Output:
1. Hello 2. Hello 3. Hello 4. Hello 5. Hello
Explanation
- The variable
iis initialized with value1. - The condition
i <= 5is checked before every iteration. - If the condition is true, the loop body prints the value of
iwithHello. - The statement
i++increases the value ofiby1. - When
ibecomes6, the condition becomes false and the loop stops.
Example: Sum of First 5 Numbers
This example calculates the sum of numbers from 1 to 5.
public class SumWhileLoop {
public static void main(String[] args) {
int i = 1;
int sum = 0;
while (i <= 5) {
sum = sum + i;
i++;
}
System.out.println("Sum = " + sum);
}
}Output:
Sum = 15
Example: Reverse Counting using while Loop
A while loop can also be used to count backward.
public class ReverseWhileLoop {
public static void main(String[] args) {
int i = 5;
while (i >= 1) {
System.out.println(i);
i--;
}
}
}Output:
5 4 3 2 1
Infinite while Loop
An infinite loop happens when the condition never becomes false. This usually happens when the update statement is missing or incorrect.
public class InfiniteWhileLoop {
public static void main(String[] args) {
int i = 1;
while (i <= 5) {
System.out.println(i);
// i++ is missing, so this loop never ends
}
}
}false. Otherwise, the while loop may run forever.
Difference Between while Loop and for Loop
| while Loop | for Loop |
|---|---|
| Used when the number of iterations is not always known. | Used when the number of iterations is usually known. |
| Initialization, condition, and update are written separately. | Initialization, condition, and update are usually written in one line. |
| Good for condition-based repetition. | Good for counter-based repetition. |
Common Mistakes in while Loop
- Forgetting to update the loop variable.
- Writing a condition that never becomes false.
- Using
=instead of==in conditions. - Adding an unwanted semicolon after the while condition.
- Changing the loop variable incorrectly inside the loop.
- The
whileloop repeats code while the condition is true. - It is an entry-controlled loop because the condition is checked first.
- If the condition is false initially, the loop body does not execute.
- The loop should include a proper update to avoid infinite loops.
- while loop is useful when the number of iterations is not fixed in advance.
Interview Questions ⭐
?? Test your understanding with a quick quiz
🚀 Quick Knowledge Check
Topic: While-loop | Language: Java
Q1. When is the condition checked in a while loop?
Q2. What will be the output?
int i = 5;
while (i >= 1) {
System.out.println(i);
i--;
}Q3. Which statement usually prevents an infinite while loop?
Q4. What is the main purpose of a while loop in Java?
Q5. Which of the following is true about a while loop?
Q6. Which loop is usually preferred when the number of iterations is not known in advance?
Q7. What will be the output of this code?
int i = 1;
while (i <= 3) {
System.out.println(i);
i++;
}Q8. What will happen in this code?
int i = 1;
while (i <= 5) {
System.out.println(i);
}Q9. Which keyword is used to write a while loop in Java?
Q10. Can a while loop execute zero times?
?? Great job! Continue learning Java step by step.
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